1. Field of the Invention
The present invention relates to a vari-focal lens control system. More particularly, the invention relates to a system for controlling a vari-focal lens having a magnification change photographic optical system comprising a magnification changing lens group and a focusing lens group disposed on a single optical axis, which may have an imaging offset with a single subject after the focusing lens group is set at a focusing point between the closest and the infinity positions on the optical axis corresponding to a subject distance ranging from the closest to the infinity distance and when an overall system focal length of the magnification change photographic optical system is renewed by the magnification changing lens group from an arbitrary first focal length to a second focal length between the shortest focal length and the longest focal length.
2. Description of the Related Art
A focusing (operation to make an image in focus) is usually effected in a zoom lens by moving a focusing lens group provided in a magnification change optical system. Such a zoom lens has an advantage that a distance of movement of the focusing lens group is fixed for the same subject distance in an entire zoom range. This is referred to as an "equivalent movement". Accordingly, a scale of subject distance is attached to a moving member (distance ring) of the focusing lens group, and an index to a fixed ring disposed adjacent to the distance ring, without a need to change a scale of subject distance during zooming.
However, although a situation differs depending upon a lens arrangement of the magnification change optical system, the lens arrangement is likely to become complicated with an optical design under a condition that the above equivalent movement can be effected in a zoom lens of inner focusing method or of rear focusing method. There is another problem in such a zoom lens that a distance of movement (feed length) of the focusing lens group becomes unnecessarily greater on the wide angle side.
Further, that results in increase in outer diameter of lens, increasing the weights of lens and lens barrel.
There is a vari-focal lens already proposed without a condition of the equivalent movement to solve the above problems. Nonetheless, the vari-focal lens has a problem of imaging offset during a magnification change operation, which corresponds to a zooming operation in the above zoom lens.
It is considered to solve this problem that a part of magnification change optical system constituting the vari-focal lens is arranged to be a focusing lens group for focusing drive independent of the magnification change operation, that an imaging offset, which will also be referred to as a shift, during the magnification change operation is calculated by calculation, and that a position of the focusing lens group is corrected based on the calculation result. Such an arrangement permits an operability substantially equivalent to that of the zoom lens.
For the automatic shift correction, a control system is for example constituted by a magnification change drive unit for driving the magnification change optical system by a magnification change motor or the like to set an arbitrary focal length, a focusing drive unit for driving the focusing lens group by a focus motor or the like, a focal length detector such as a potentiometer for detecting a presently set focal length as an analog quantity such as a voltage, and a focus position detector such as a potentiometer for detecting a current position of the focusing lens group as an analog quantity such as a voltage.
In the above vari-focal lens, a focusing position of the focusing lens group changes depending upon a focal length with an identical subject. A locus of the change is a hyperbola with a variable of focal length (an output of the focal length detector). Suppose the focusing lens group is in focus and the magnification is changed from this state. The control system of the vari-focal lens keeps the in-focus state by moving the focusing lens group along the hyperbola, such that a focal length to be set may be reached to by alternately repeating a focusing operation (as will be hereinafter referred to as a shift correction operation) and a magnification change operation changing the focal length in order to naturally correct a change in view angle in a finder.
A next feed length and drive direction of the focusing lens group to an in-focus position are calculated for each shift correction operation as alternately repeated.
It is desirable that a still camera of single lens reflex type or a video camera using such a vari-focal lens always keeps the in-focus state during the magnification change. Because a still camera can execute a photographing efficient and compatible to a photographic intention by keeping a screen observed in focus during magnification change. Also, a video camera is naturally required to have no focus deviation during photographing with a magnification change.
There are two major concrete techniques for keeping in-focus condition during magnification change.
Japanese Unexamined Patent Publication No. 68-88534 describes the first technique, in which a subject distance is detected immediately before a start of magnification change, and a position of a focusing lens group or an image pickup device forming a magnification change photographic optical system is controlled in drive by an operational circuit such that the subject distance detected is kept unchanged during magnification change.
Japanese Unexamined Patent Publication No. 62-231207 describes the second technique, in which a magnification change position datum is obtained by detecting a position of a magnification changing lens group or a member moving with the magnification change operation, forming a magnification change photographic optical system which has once been brought into an in-focus state by known means, and in which a focusing position datum is obtained by detecting a position of a focusing lens group or a member moving with the focusing operation while driving the focusing lens group forming the photographic optical system in correspondence to a subject distance.
Then the focusing lens group is controlled in drive to always keep the in-focus state even during magnification change for an arbitrary subject distance based on the two data, that is, the magnification change position datum and the focusing position datum.
In the first technique as described above, a well-known phase difference detection method using a CCD is commonly used as means for detecting the in-focus state immediately before the start of magnification change in the magnification change photographic optical system.
From another point of view, there are two methods to effect focusing correction during magnification change in a vari-focal lens.
The first method is that focusing detection is always conducted by known focusing state detection means using a CCD during magnification change and that a focusing lens group forming a photographic optical system is driven for focusing based on a result of focusing detection.
The second method is that an in-focus state is first achieved with use of known in-focus state detection means using a CCD immediately before starting magnification change, that both positions of focusing lens group and of magnification changing lens group forming a magnification change photographic optical system are detected during magnification change, and that the focusing lens group is driven to a position to be set based on the positional relation between them.
If a conventional vari-focal lens control apparatus employs the above first method for focusing correction during magnification change, the focusing correction could be nullified in case of inability of distance measurement due to a condition of subject, for example, due to a flat brightness distribution or to a specific shape of subject.
If a conventional vari-focal lens control apparatus employs the second method, and if a subject moves during magnification change to change a subject distance, the system becomes out of focus at the point of subject movement.
Comparing a responsivity to focusing correction during magnification change in the vari-focal lens control apparatus employing the first method with that in the control apparatus employing the second method, the first method requires a more time than the second method, because a charge storage time is necessary for the CCD conducting the focusing detection during magnification change. If a brightness of subject is extremely low a charge storage time for CCD becomes longer, greatly decreasing the responsivity.